Strange shader/material on all my cubes I created in blender 2.8
Unfortunately you cropped the screenshot too tight so we can't see the shading settings on top of the screen.

That shading you see is just to help you see objects better in the 3D viewport, specially in solid mode, where you don't have materials to help you and it would be hard to distinguish shapes otherwise.

You have some control on how things are presented on the screen by clicking on the controls for shading. For solid view you can choose from several presets Flat, Studio or Matcap.
Among the setting and you can also choose to rotate the light source. In matcaps you can see the object with different textures.



For DevLook,is the same, you can visualize your object under different environments, or the actual light from the scene.



Note that this is not necessarily faithful representations of materials or lighting. Depending on your choice of shading for the viewer you might see a material that has nothing to do with the actual materials, but will help you visualize your object. will render or how the lights on the scnene are working. The light on final render will depend on the light sources and the environment. To make an assessment of the real materials and the actual lights of the scene use Rendered view.